1. Introduction
This invention relates to electroless metal deposition and more particularly, to catalytic compositions useful therefor.
2. Description of the Prior Art
Electroless metal deposition refers to the chemical deposition of a metal on a conductive, non-conductive, or semi-conductive substrate in the absence of an external electric source.
Electroless deposition is used for many purposes, for example, in the manufacture of printed circuit boards where, in one method, an electroless metal, typically copper, is deposited on a dielectric substrate either as a uniform surface coating or in a predetermined pattern. This initial electroless deposit is usually thin and may be further built up by electroplating or may be deposited directly to full thickness.
The substrate over which an electroless metal deposit is formed is most often a plastic panel which may have a metal foil such as copper laminated to one or both of its surfaces, for example, with adhesive, to form a metal clad substrate. Where both surfaces of the substrate are to be used, connections are typically provided therebetween by means of holes through the panel at appropriate locations, the walls of these holes being made conductive with the electroless coating.
The electroless deposition of a metal on either a metallic or non-metallic substrate usually requires pretreatment or sensitization of the substrate to render it catalytic to the reception of such a deposit. Various methods have evolved over the years employing particular sensitizing compositions.
An early method of sensitization involves plural baths wherein a substrate is subjected first to immersion in an acidic stannous chloride solution and, following water rinsing, immersion in an acidic palladium chloride solution whereby the palladium chloride is reduced to a catalytic layer of palladium metal. This method has severe limitations because the palladium chloride solution, in contact with the copper layer of a copper clad substrate, forms an immersion deposit which is loosely adhered to the copper. This is wasteful of palladium and interferes with the bond between the copper cladding and a subsequently deposited electroless metal layer.
A major advance in the art of electroless metal deposition was achieved by the compositions and processes of U.S. Pat. No. 3,011,920 incorporated herein by reference. According to said patent, a substrate was catalyzed by treatment with a liquid which contained a colloidal dispersion of a catalytic metal formed by the admixture of catalytic metal ions and stannous ions in an amount in excess of the catalytic metal ions in an acidic aqueous solution. The colloid is believed to be of catalytic reduced metal having a protective colloid comprising stannic acid. By this method, the immersion coating formed by contact of palladium ions with copper cladding was eliminated as the catalytic metal is not in an ionic form, but rather, is in colloidal form, where the colloid is believed to be in an insolubilized reduced form of the catalytic metal. Other advantages were achieved with the invention of U.S. Pat. No. 3,011,920 such as decreased cost resulting from reduction in the amount of catalytic metal consumed, improved coating, improved bond strength between coating and substrate, greater reliability and other improvements as enumerated in said patent.
In U.S. Pat. No. 3,672,938, there is disclosed a process for catalyzing a substrate prior to electroless metal deposition and catalyst compositions therefor which compositions are also formulated by the admixture in acid solution of a catalytic metal salt, a stannous salt in molar excess of the catalytic metal salt and a hydrohalide acid. This catalyst is said to differ from the catalysts of U.S. Pat. No. 3,011,920 in physical form, it being asserted that the catalyst of said patent is an optically clear "true solution catalyst" of unreduced catalytic metal in complex form rather than a colloidal catalyst of reduced catalytic metal as in the aforesaid U.S. Pat. No. 3,011,920.
Improvements over catalysts disclosed in U.S. Pat. No. 3,011,920 and 3,672,938 are disclosed in U.S. Pat. Nos. 3,874,882 and 3,904,792. The inventions of these latter patents are predicated upon the discovery that urea addition to such catalysts minimizes stannous ion oxidation loss and consequently, catalyst instability; and halide ions play a significant roll in the functioning of such catalysts, the catalyst being improved when the concentration of halide ions is increased beyond that concentration found in the prior art catalysts by the addition of an extraneous source of halide ions. Therefore, the improvements resulting from the addition of urea to catalyst formulations and/or excess halide ions comprise improved stability and adsorption properties, and solubilization of the stannous salt or retardation of the point wherein the catalyst coagulates and precipitates. Accordingly, catalysts of increased stability and pH can be formulated thereby providing catalysts suitable for use with materials readily attacked by strong acids.
For purposes of economy in shipping and storage, it is desirable to provide a catalyst in dry form which can be redispersed by simple admixture with an aqueous acidic solution. With reference to the catalysts of U.S. Pat. No. 3,011,920, it was believed that the catalysts thereof could not be dried and redispersed. This belief was due in part to stability tests where catalyst was left exposed in an open container with a large surface area, such as a petri dish. In these tests, it was found that with standing and evaporation of the liquid components of the catalyst, the catalyst would coagulate prior to complete drying and once coagulated, could not be redispersed.
In the prior art, one attempt at production of a dry catalyst material is reported in U.S. Pat. No. 3,672,923 wherein the catalysts of U.S. Pat. No. 3,672,938 are produced in dry form. These dry catalysts, and the liquid catalysts from which they are formed, are reported to be complex, optically clear, non-colloidal "true solution catalysts" of unreduced catalytic metal ions when in aqueous acidic medium as distinguished from colloidal catalysts. In said U.S. Pat. No. 3,672,923, it is reported that because the catalysts are true solution complexes, they can be dried to powder form whereas a colloidal catalyst could not be similarly dried. A typical reported method for making a complex catalyst, as distinguished from a colloidal catalyst, and drying the same is set forth in Example 3 of the patent wherein stannous chloride is dissolved in 37% hydrochloric acid, diluted, and admixed with palladium chloride. This mixture is heated to 85.degree. C for 20 minutes, then boiled for 1.5 hours, cooled and vacuum evaporated to dryness. It is reported that this procedure forms a true solution complex catalyst.
In copending U.S. Patent Application Ser. No. 618,033 filed contemporaneously herewith, dry catalyst compositions are reported that upon dispersion in an aqueous acidic solution of pH below about 1, have substantially the composition of the acidic catalysts of U.S. Pat. No. 3,011,920. These catalytic compositions, both in dry and in suspended form, are believed to be of reduced colloidal catalytic metal. The dry compositions are made either from the liquid catalysts compositions of said U.S. Pat. No. 3,011,920 using methods that avoid aerial oxidation of stannous tin or by replenishing dry catalyst compositions that have undergone aerial oxidation with an additional stannous salt, or the dry catalyst composition may be made directly from the solid components of the catalyst in dry form.